The invention relates to motion-picture cameras provided with an automatic diaphragm-control system, the photosensitive element of which receives light actually passing through the objective of the camera. In particular, the invention relates to such cameras which are furthermore provided with means for implementing image fadeovers. An image fadeover comprises an image fade-out followed by an image fade-in. The fade-out is implemented by applying an imbalancing effect to the diaphragm-control system, causing the diaphragm aperture size to decrease. The subsequent fade-in is implemented by removing the imbalancing effect, so that the diaphragm aperture decrease is removed. The amount of the aperture-size decrease for fade-out and subsequent increase for the fade-in is conventionally a predetermined amount.
Often, in such systems, the aperture-error signal is an analog signal, but the analog signal is then evaluated to produce a digital signal which then actually controls the operation of the diaphragm-adjusting motor; i.e., when the adjusting motor is to operate, the control signal applied to it is a simple on-off signal not containing analog aperture-error information.
Such conventional motion-picture cameras have the advantage that, when the system is deliberately imbalanced to implement a fade-out, the amount by which the aperture size then decreases is constant and independent of what the aperture size happened to be at the moment the fade-out was commanded. Furthermore, the scene-light-dependent negative-feedback action of the diaphragm control system continues, although with the fade-out aperture-size decrease superimposed upon it.
In particular, when the control signal for the diaphragm-adjusting motor is not analog but digital, the rate at which the aperture size changes during fade-ins and fade-outs is predetermined, and the total duration of the fade-out or fade-in is independent of the prevailing value of scene-light intensity. When the aperture-size has been decreased for such an effect, the amount of light reaching the through-the-lens photosensitive element of the diaphragm control system is very low. As a result, if one uses a photoresistor as the photosensitive element, its sluggishness in responding to scene-light changes becomes problematic. Additionally, its sluggishness of response can vary in dependence upon its recent history; e.g., the response sluggishness of a photoresistor which has just previously been exposed to no light at all or extremely low light is much greater than if the photoresistor had just been previously been exposed to a comparatively higher amount of light.
If one uses instead a silicon photodiode, then one must deal with the disadvantage of considerably lower photocurrent than is achieved with a photoresistor. When a silicon photodiode is used, and the diaphragm is in its aperture-size-decreased state, the photocurrent may be on the order of only pico- or nanoamperes, requiring considerable expense for amplification. A further problem relates to the fact that the dark current (zero-incident-light current) of such a photodiode will constitute a sizable fraction of its total current.